Chordal gas flow meter with transducers installed outside the pressure boundary

ABSTRACT

A flowmeter for detecting gas flow rates in a pipe includes a container configured to be attached to the pipe having a channel through which the gas flows, and a plurality of recesses that extend through the container and a plurality of housings, each recess having a housing. The flowmeter includes a plurality of transducers, with one transducer of the plurality of transducers disposed in each housing in each recess, the transducers transmitting ultrasonic signals into and receiving ultrasonic signals from the channel. The flowmeter includes acoustic isolators which acoustically isolate the housings from the container. The flowmeter includes a controller in electrical communication with the plurality of transducers which determines the gas flow rate through the channel by measuring transit times of signals transmitted by and received by the transducers. A method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 14/031,837filed Sep. 19, 2013, now U.S. Pat. No. 9,322,689, which is acontinuation application of U.S. patent application Ser. No. 12/927,616filed Nov. 19, 2010, now U.S. Pat. No. 8,544,343, incorporated byreference herein.

FIELD OF THE INVENTION

The present invention is related to a flowmeter that determines gas flowrates in a pipe by means of ultrasonic transducers that send and receivesignals into and from the gas flow through a window made of a pressurecontaining material that is in acoustic communication with a channel inwhich the gas flows. (As used herein, references to the “presentinvention” or “invention” relates to exemplary embodiments and notnecessarily to every embodiment encompassed by the appended claims.)More specifically, the present invention is related to a flowmeter thatdetermines gas flow rates in a pipe with ultrasonic transducers thatsend and receive signals into and from the gas flow through a windowmade of a pressure containing material that is in acoustic communicationwith a channel in which the gas flows where the transducers are disposedin housings that acoustically isolate the transducers so as to improvethe signal-to-noise ratio of the received acoustic signal.

BACKGROUND OF THE INVENTION

This section is intended to introduce the reader to various aspects ofthe art that may be related to various aspects of the present invention.The following discussion is intended to provide information tofacilitate a better understanding of the present invention. Accordingly,it should be understood that statements in the following discussion areto be read in this light, and not as admissions of prior art.

Ultrasound transducers are used to transmit and receive ultrasonicsignals in a flowmeter system. Several problems were solved by using thepresent invention, they are: The ability to remove or repair transducersfrom existing ultrasonic flow meters required either depressurizing thepipe gas line or the use of special tools for transducer replacement;transducers not in direct contact with the gas contained in the pipeusually have poor transducer performance resulting in poor signaldetection for gas meters. Current ultrasonic gas meters use gas “wetted”transducers, that is, the transducer itself is in direct contact withthe gas and cannot be removed without special tools or the eliminationof the pressure in the pipe. Traditional transducer housings that putthe transducer outside of the pipe pressures, such as those are used inliquid meters, have not been used since the acoustic losses through thehousings combined with the noise produced by the mounting of thetransducer housings make the acoustic signals unusable. The traditionaltransducer housings do not work in a gas environment for two reasons.First, conventional transducer housings had thick metal windows thatpoorly match the acoustic impedance of gas and therefore would nottransmit sound into the gas. Second, the traditional transducer housingsare rigidly attached to the meter body. These rigid attachments providepathways for sound to be transmitted through the meter body and notthrough the gas resulting in poor signal to noise ratios.

The current ultrasonic gas meters have their wetted transducers exposedto the gas which can contain hydrogen sulfide or other contaminants.Hydrogen sulfide can over time deteriorate transducers made ofconventional solder joints and epoxies for electrical and mechanicalconnections within the transducer. In current ultrasonic gas meters, ametal seal is placed behind the transducer in order to maintain the pipepressure and to prevent gas from leaking out. So if a transducer fails,the transducer has to be replaced using special tools that prevent thetransducer from bursting out of the flowmeter at high speeds due to pipegas pressure. This may be deadly to the replacer if the tools areimproperly handled because the transducer becomes a projectile underpipe gas pressure. In addition, the escaping gas is usually highlyflammable, presenting an extreme hazard to the replacer and othersnearby. If the gas pipeline/process is depressurized to replacetransducers for safety reasons the pipeline flow ceases causing lostrevenue.

Current gas meter transducers use either a monolithic PZT ceramictransducer or a Tonpilz transducer. These transducers suffer from poorbandwidth, poor signal to noise ratio and radial modes in the 100 to 300kHz frequency range, the preferred ultrasonic operating frequency rangefor gas meters. As a result, the received signals can be very distorted.This results in poor transit time measurements and poor accuracy of agas flowmeter Current gas meters also have metal to metal contactbetween parts even with wetted transducers when cases enclosing thetransducers are made of metal so the flow meter can suffer from poorsignal to noise ratio due to acoustic noise of the system.

BRIEF SUMMARY OF THE INVENTION

The present invention pertains to a flowmeter for measurement of gaswhere accuracy and reliability are essential. The flowmeter measures gasflow rates in a pipe having a channel disposed in the pipe through whichgas in the pipe flows and plane waves generated by multiple upstreamultrasonic transducers and multiple downstream ultrasonic transducerspropagate. The arrangement of the transducers defines two crossingplanes but may have more or fewer. An important feature of this meter isthat the transducer elements can be safely checked or replaced withoutspecial tools and without depressurizing the line because of housingswhich hold the transducer and contain the pipe pressure.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

In the accompanying drawings, the preferred embodiment of the inventionand preferred methods of practicing the invention are illustrated inwhich:

FIG. 1 shows a flowmeter of the present invention.

FIG. 2 shows a flowmeter top section of two crossing planes (A and B).

FIG. 3 shows a flowmeter cross section of along either Plane A or PlaneB.

FIG. 4 shows a gas meter arrangement.

FIG. 5a shows a gas transducer.

FIG. 5b shows an exploded view of a gas transducer.

FIG. 6 shows the transformer application.

FIGS. 7a and 7b , which together are one continuous drawing, show ademonstration of transit time flow meter performance.

FIGS. 8a and 8b , which together are one continuous drawing, show errorvs. velocity in regard to the claimed invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings wherein like reference numerals refer tosimilar or identical parts throughout the several views, and morespecifically to FIGS. 1-4 thereof, there is shown a flowmeter 10 fordetecting gas flow rates in a pipe 12. The flowmeter 10 comprises acontainer 11 configured to be attached to the pipe 12 having a channel17 through which the gas flows, and a plurality of recesses 15 thatextend through the container 11 and a plurality of housings 14. Eachrecess 15 has a housing 14 which contains pressure in the channel 17.Each housing 14 has a window 24 that is in acoustic communication withthe channel 17. The flowmeter 10 comprises a plurality of transducers32, with one transducer 32 of the plurality of transducers 32 disposedin each recess 15. The transducers 32 transmit ultrasonic signals intoand receive ultrasonic signals from the channel 17 through the window 24in the housing 14 in which a transducer 32 is disposed. The flowmeter 10comprises a controller 20 in electrical communication with the pluralityof transducers 32 which determines the gas flow rate through the channel17 by measuring transit times of signals transmitted by and received bythe transducers 32.

The window 24 may have a thickness less than ¼ wavelength of ultrasoundin the window's material. The window 24 thickness may be about 1/10wavelength of ultrasound in the window's material. One transducer 32 ofthe plurality of transducers 32 may be disposed in each housing 14disposed in each recess 15. The housing 14 may be a pressure boundarywhich contains the pressure in the channel 17 and essentially preventsgas in the channel 17 from escaping into the housing 14. The housing 14forms a gas tight seal with the channel 17.

The transducers 32 may be removed from the recesses 15 without having todepressurize the pipe 12 or having to use an extraction tool thatremoves the transducers 32 through a pressure containing component thatwould contain the pressure in the channel 17. The ultrasonic signalstransmitted and received by the transducers 32 may define a first pathin a first plane and a second path in a second plane which paths crossin the channel 17.

The flowmeter 10 may include acoustic isolators 22 which acousticallyisolate the transducer housings 14 from the container 11. The transducer32 may couple to the window 24. The window 24 is made of metal orplastic. The window 24 may be made of titanium, PEEK or PPS.

Each transducer 32 may be a broad band piezoelectric compositetransducer 32 with a coupling coefficient greater than 0.7 out of 1 andan acoustic impedance of less than 34 Mrayls.

The present invention pertains to a housing 14, as shown in FIG. 4, foran ultrasonic transducer 32 for a flowmeter 10 which is inserted into arecess 15 of a container 11 through which gas flows. The housing 14comprises a shell 25 in which the transducer 32 is disposed. The shell25 has an outer surface and a flange 34 that extends from the outersurface of the shell 25 and a plurality of acoustic ribs 26 disposed inproximity to an end of the shell 25 from which signals are emitted bythe transducer 32. The shell improves the signal-to-noise ratio togreater than 100:1, the ribs attenuate all non-gas paths of sound by atleast 25%.

The housing 14 may include an acoustic isolator 22 disposed about thehousing 14, which isolator contacts the container 11 and the housing 14when the transducer 32 is disposed in the recess 15 of the container 11,with the acoustic isolator 22 disposed between the container 11 and thehousing 14 so the housing 14 does not contact the container 11. Theacoustic isolator 22 may include discs disposed about the flange 34 ofthe housing 14. The discs may be made of plastic, syntactic foam orrubber. Impedance matching material may not be used on the exteriorsurface to improve transmission of signals from the transducer 32 intothe gas, but uses a window 24 thickness less than ¼ wave length, whichmakes the window 24 acoustically transparent. There may be no metal tometal contact between the housing 14 and the container 11.

The present invention pertains to a method for detecting gas flow ratesin a pipe 12. The method comprises the steps of transmitting ultrasonicsignals from a plurality of transducers 32 disposed in recesses 15 in acontainer 11 attached to the pipe 12 through a window 24 made of metalof each recess 15 into a channel 17 of the container 11 in which the gasflows, with one transducer 32 of the plurality of transducers 32disposed in each recess 15 and the window 24 in acoustic communicationwith the channel 17. There is the step of receiving ultrasonic signalsfrom the channel 17 through the windows 24 by the transducers 32 in therecesses 15. There is the step of determining the gas flow rate throughthe channel 17 by measuring transit times of the signals transmitted byand received by the transducers 32 with a controller 20 in electricalcommunication with the plurality of transducers 32.

The window 24 may be a pressure boundary which contains the pressure inthe channel 17 and prevents gas in the channel 17 from escaping into thehousing 14, the window 24 forming a gas tight seal with the housing 14,and there may be the step of replacing a transducer 32 withoutdepressurizing the channel 17 or without using an extraction tool thatremoves a transducer 32 through a pressure containing component thatwould contain the pressure in the channel 17.

The transmitting step may include the step of transmitting ultrasonicsignals by the transducers 32 along a first path in a first plane and asecond path in a second plane which cross in the channel 17 and thereceiving step may include the step of receiving ultrasonic signals bythe transducers 32 from the first path and from the second path. Theremay be the step of acoustically isolating housings 14 in which thetransducers 32 are disposed in the recesses 15 from the container 11with acoustic isolators 22. There may be the step of applying a forceagainst the transducer 32 to hold the transducer 32 in contact with thewindow 24.

The transmitting step may include the steps of generating with anupstream ultrasonic transducer 32 of the first path plane waves thatpropagate through the channel 17 and are received by a downstreamultrasonic transducer 32 of the first path; producing a downstreamtransducer 18 signal with the downstream transducer 18 from the planewaves the downstream transducer 18 receives; generating with thedownstream ultrasonic transducer of the first path plane waves thatpropagate through the channel 17 and are received by the upstreamultrasonic transducer of the first path; producing an upstreamtransducer 16 signal with the upstream transducer 16 from the planewaves the upstream transducer 16 receives; and determining with thecontroller 20 the gas flow rate from transit times of the signalsgenerated and received by the upstream transducer 16 and downstreamtransducer 18.

The replacing step may include the step of replacing the transducer 32without using an extraction tool that removes the transducer 32 througha pressure containing component that provides a gas tight,pressure-bearing enclosure, within which the transducer 32 can beunfastened from the container 11 and allowing the gas in the channel 17to fill the enclosure without leaking into an external environment aboutthe container 11. The replacing step may include the step of replacingthe transducer 32 without using a valve to seal the recess 15 from whichthe transducer 32 is removed thereby allowing the enclosure within theextraction tool to be vented.

Unlike the present invention, in a design in which the transducer itselfis immersed in the gas, the extraction tool performs the followingfunctions:

-   -   1. It provides a gas tight, pressure-bearing enclosure, within        which the exterior of the transducer assembly can be unfastened        from the container (thereby breaking the normal pressure        barrier) and allowing the enclosed gas to fill the extraction        tool enclosure without leaking into the external environment.    -   2. It provides the means to withdraw the transducer assembly        within the extraction too.    -   3. It provides the means—usually a valve—to close off the        opening left vacant by the removal of the transducer assembly,        thereby allowing the enclosed space within the extraction tool        to be vented and the enclosed (presumably defective) transducer        assembly to be removed.    -   4. It provides the means to put in place a fully functional        transducer assembly within the enclosed space of the extraction        tool.    -   5. It provides the means to insert the new transducer into the        container, fasten it in place and test the transducer        assembly/container joint for tightness, whereupon the extraction        tool can be removed.

The present invention pertains to a transducer 32, as shown in FIG. 5,for an ultrasonic flowmeter 10. The transducer 32 comprises a case 58.The transducer 32 comprises a broad band piezoelectric composite 50disposed in the case 58 with a coupling coefficient greater than 0.7 outof 1 and an acoustic impedance of less than 34 Mrayls.

The transducer 32 may include a transformer 30 which matches thetransducer's electrical impedance.

In another embodiment, the housing 14 and transducer 32 describedherein, as well as the overall technique described herein, may beapplied directly to a pipe 12 where the recesses 15 are formed in thepipe 12 and the housings 14 with transducers 32 are inserted into therecesses 15 in the pipe 12 itself.

In the operation of the invention, the flowmeter 10 is capable ofmeasuring gas flow rates with ultrasonic transit time technology. Theapplication is specifically applied to natural gas metering. A novelaspect of the flowmeter 10 is the transducer housing 14 which has apressure containing window 24 less than 1/10 of a wavelength of theultrasound of the window 24 material in thickness but neverthelesscomplies with the strength and tightness requirements imposed by thefull pressure of the gas on its exterior. The preferred window 24 is atitanium window 24. The titanium window 24 is thin enough such that itis nearly acoustically transparent, it serves as a hermetic seal fromthe natural gas which may contain hydrogen sulfide or othercontaminants, and it acts as a pressure barrier to the gas in the pipe12. A wavelength (λ) of titanium at 200 kHz is 30.35 mm when the speedof sound in the material is 6070 m/s. The titanium window 24 is laserwelded to the titanium transducer housing 14. Laser welding is a lowheat process compared to torch welding therefore there is no damage tothe mechanical integrity of the λ/10 thick (3 mm) titanium window 24 tothe housing 14. The window 24 serves to maintain the mechanicalintegrity of the transducer housing 14 by being a pressure barrier fromthe pressurized gas. Typical operation pressure ratings range from below475 psi (ANSI 150) to 3,705 psi (ANSI 1500). The operating temperaturesrange from −40 C to 100 C. The gas flow meter has replaceabletransducers 32 without having to depressurize the line. All transducerhousings 14 have acoustic ribs 26 which are spaced λ/8 to λ/4 apart toisolate and cancel the acoustic noise of the transducer housing 14 andbreakup the transducer housing 14 resonance. In addition, all transducerhousings 14 are acoustically isolated from the meter body by acousticisolators 22.

Special transducers 32 have been fabricated using compositepiezoelectric technology. The composite piezoelectric material has ak_(t) of 0.75 and low Z of 17 MRayls. This enables broad bandperformance for an easily detectable leading edge for precise timemeasurements. Each transducer 32 uses a 7:1 impedance matchingtransformer 30 which improves signal strength by 17 dB. This transducerarrangement coupled to a titanium window 24, λ/10 is desired forperformance of an ultrasonic gas meter that uses a window 24 as apressure barrier greater than 3,705 psi.

A flowmeter 10 for detecting gas flow rates in a pipe 12 preferablyincludes multiple paths disposed in the pipe 12 through which gas in thepipe 12 flows. The upstream transducer 16 is in contact with thetitanium window 24 within the transducer housing 14 and positioned suchthat plane waves generated by the upstream transducer 16 propagatesthrough the channel 17. The downstream transducer 18 is acousticallyisolated from the pipe 12 and positioned such that plane waves generatedby the downstream transducer 18 propagate through the channel 17 and arereceived by the upstream transducer 16, which produces an upstreamtransducer 16 signal that is provided to a controller 20. The downstreamtransducer 18 receives the plane waves from the upstream transducer 16,and provides a downstream transducer 18 signal that is provided to thecontroller 20. The flowmeter 10 includes a signal processor, otherwiseknown as the controller 20, in communication with the upstream anddownstream transducers 16, 18 which determines the gas flow rate throughthe channel 17 by measuring transit times of signals transmitted by andreceived by the transducers.

Referring to FIGS. 1-3, the flowmeter 10 comprises a channel 17 disposedin the pipe 12 through which gas in the pipe 12 flows. The flowmeter 10comprises multiple paths. Multiple upstream ultrasonic transducers 16within transducer housings 14 acoustically isolated from the pipe 12 andpositioned so plane waves generated by an upstream transducer 16propagate through the channel 17. Each path comprises a downstreamultrasonic transducer 18, within a transducer housing 14 acousticallyisolated from the pipe 12 and positioned so plane waves generated by thedownstream transducer 18 propagate through the channel 17 and arereceived by the upstream transducer 16 which produces an upstreamtransducer 16 signal. The downstream transducer 18 receives the planewaves from the upstream transducer 16 and provides a downstreamtransducer 18 signal. The flowmeter 10 comprises a controller 20 incommunication with the upstream and downstream transducers 16, 18 whichdetermines the gas flow rate through the channel 17 by measuring transittimes of signals transmitted by and received by the transducers.

The transducer housing 14 is preferably made of titanium which iscorrosion resistant to natural gas contaminants such as hydrogensulfide. The upstream transducer 16 and the downstream transducer 18 arecoupled to a window 24 within the transducer housing 14. The transducerhousings 14 are acoustically isolated by acoustic isolators 22 whichcontact the pipe 12. The window 24 forms a seal with the transducerhousing 14 preventing gas in the pipe 12 leaking into the transducerhousing 14. The window 24 which is inclusive of the transducer housing14 is sealed via laser welding such that hydrogen sulfide cannot leakinside. The transducer 32 couples to the acoustic window 24 using acouplant, preferably a silicone grease. A spring assembly 28 appliespressure to the transducer 32 in order to couple the ultrasonic signalmore effectively to the window 24. A spacer 38 and compression nut 40compress the spring assembly 28 such that at least 100 psi is applied tothe transducer 32. O-rings 42 are placed around the transducer housing14 and a bushing 44 in order to create another gas tight seal betweenthe transducer housing 14 and the pipe 12. The bushing 44 providesmechanical support for the transducer housing 14 and positions thetransducer housing 14 within the container 11. The load nut 46 and locknut 48 secure the transducer housing 14 to the container 11 by applyingpressure to the acoustic isolators 22 and prevents the transducerhousing 14 from being forced out of the container 11 under pipepressure. There is a gas gap radially disposed between the transducerhousing 14 and load nut 46 and lock nut 48 so there is no metal to metalcontact between the container 11 and the transducer housings 14. Itshould be noted that basically, the container 11 with the channel 17 isan extension of the pipe 12 and for all intent and purpose in regard tothis invention is considered a part of the pipe 12.

Shown in FIG. 5 is a transducer 32 consisting of a wearface 56,piezocomposite 50 and delay line 36 that is potted in a brass case 58with a load cylinder 52 and a cap 54. The wearface 56 is made ofimpedance matching material preferably of high purity alumina (Al₂O₃)greater than 96% with a density of around 3.64 gm/cm³, a hardnessgreater than 1000 Knoops and the thickness <<λ. The piezocomposite 50 isa 1-3 composite material preferably made from PZT-5H and epoxy (SmartMaterial Inc., Sarasota, Fla.). The volume fraction of PZT-5H is around50% this produces a high coupling coefficient, k_(t) of 0.75 out of 1.Having a high coupling coefficient is necessary because it improves thesignal to noise ratio of the ultrasonic signal. The acoustic impedanceknown as Z=ρv, where density ρ (kg/m³) and velocity v (m/s), is around17 MRayls. The delay line 36 serves two purposes: as a reflection delayline 36 and as an acoustic attenuator. The delay line 36 is made ofmetal filings filled with epoxy of at a low volume fraction, typically15%. The delay line 36 is long enough such that the reflection is morethan several wavelengths away from the piezoelectric element. A typicalspeed of sound for the delay line 36 is 1277 m/s. A wave length in adelay line 36 at 200 kHz is 6.3 mm. A typical length of a delay line 36for this application is thirty two mm. Therefore, once a signal isreceived by the transducer 32 it takes a reflection twice the time of adelay line 36 before the reflection is received as reverberation by thetransducer 32. In this case the reflection occurs fifty microsecondslater than the detectable signal. Attenuation in the delay line 36 istypically, 2.29 dB/cm, so round trip attenuation of a reflected signalwould be 14.65 dB. In order to make electrical connections topiezocomposite 50, a (+) wire 60 and a (−) wire 62 are soldered tosilver foil which is bonded to the piezocomposite 50 using silver epoxy.The entire transducer 32 is potted with non-conductive epoxy in order toencapsulate and insulate all components.

Depending on the specific operating conditions of the meter, a range offrequencies can be used, from 100 kHz to 500 kHz. A broad band compositetransducer 32 is preferred for metering gas. Unique to gas applications,when flow rate >60 feet/second is present the acoustic wave is distortedby the flow and the wave front is received by a transducer 32. If abroad band composite transducer 32 is used instead of a typical narrowband monolithic PZT transducer or Tonpilz transducer there is lessdistortion of the received acoustic wave. The upstream and downstreamtransit time signals are detected in order to make a flow measurement bythe controller 20. The composite transducer 32 has a rising edge of thereceived ultrasonic pulse that can be more accurately detected by thecontroller 20 thus calculating a more accurate transit time measurement.

An electrical transformer 30 is put in series with each transducer 32 tomatch the electrical impedance of the transducer 32 to that of theelectronics and cables, FIG. 6. The electronics are represented by an ACvoltage V in series with a transformer, L₁, the first winding of thetransformer and the impedance Z of a transducer is in series with thesecondary winding of the transformer, L₂. The equivalent circuit isshown when the electronics, V, are directly in series with the impedanceZ of the transducer and the impedance is reduced by the number of turnssquared, N². The electrical impedance of each transducer 32 (4900Ω) isreduced N² by 49 to 100Ω in order to match to the 100Ω transmit andreceive electronics. Each transducer 32 is spring loaded by the springassembly 28 inside the transducer housing 14 in order for the wearface56 to make sufficient mechanical contact to the window 24. Grease isused for acoustic coupling of the transducer 32 to the window 24.

The transducer housing 14 has acoustic ribs 26, acoustic isolators 22,and a window 24. The acoustic ribs 26 are spaced λ/4 to λ/8 apart inorder to cancel acoustic noise. During the excitation of an ultrasonictransducer 32 which is coupled to the window 24, a low amplitudeacoustic wave either longitudinal or shear propagates along thetransducer housing 14. The wave propagates according to the waveequation:

$\frac{d^{2}y}{d\; x^{2}} = {\frac{1}{c^{2}}\frac{d^{2}y}{d\; t^{2}}}$

When a longitudinal or shear wave is reflected from a rib interface itsphase changes ω=π or 180 degrees ˜t=T/2 or x=λ/2, it then interfereswith an incoming wave to destructively cancel it. Therefore,y(x,t)=y ₁(ct−x)−y ₁(ct+x)=0

Since the ambient noise generated in the transducer housing 14 can be atmany frequencies and amplitudes, cancelation is not complete. The noisecancellation improves the signal to noise ratio of the receivedultrasound signal up 10 dB.

The acoustic isolators 22 are preferably made of Polyether Ether Ketone(PEEK) disks (thickness>λ) that fit around the transducer housing 14between a flange 34. Other low acoustic impedance materials may be used,for example syntactic foam. The acoustic isolators 22 are in contactwith the pipe 12 but the flange 34 is not. The total noise loss fromnoise attenuation can be calculated using transmission equations usingthe following formula:

${{Loss}({dB})} = {{20\mspace{14mu}\log\; 10\left( \frac{P_{o}}{P_{in}} \right)} = \frac{16*\left( Z_{peek} \right)^{2}Z_{steel}Z_{Ti}}{\left( {Z_{peek} + Z_{Ti}} \right)^{2}\left( {Z_{peek} + Z_{steel}} \right)^{2}}}$

Where P_(o) is pressure output, P_(in) pressure input, Z_(peek), theacoustic impedance of PEEK is 3 Mrayls, Z_(steel), the acousticimpedance of Steel is 45 Mrayls, and Z_(Ti), the acoustic impedance ofTitanium is 30 Mrayls. The loss calculation is 22 dB but more loss ispossible since the PEEK, steel and titanium parts are pressed togetherand not mechanically bonded.

The window 24 is less than wavelength thick such that it becomesvirtually acoustically transparent at operating frequencies. Accordingto transmission line theory the acoustic impedance of material becomescloser to that acoustic impedance the thinner it becomes. When thethickness of the titanium window l is λ/10 of titanium, the acousticenergy transmits through the window 24 into gas without signaldistortion. The equation for transmission line theory is:

$Z_{w} = {Z_{Ti}\frac{Z_{A} + {j\; Z_{Ti}{\tan\left( {\beta\; l} \right)}}}{Z_{Ti} + {j\; Z_{A}{\tan\left( {\beta\; l} \right)}}}}$

Z_(A): Acoustic Impedance of Air

Z_(Ti): Acoustic Impedance of Titanium

Z_(w): Acoustic Impedance of the window 24 of thickness l

C_(Ti): Speed of Sound in Titanium

f: Frequency of ultrasound wave

λ=C_(Ti)/f: Wavelength in Titanium

β=2π/λ

Since titanium has lower acoustic impedance than steel (typical materialof gas pipes), it is the best metal to use acoustically that isresistant to hydrogen sulfide and other contaminants.

The ultrasonic flow meter arrangement 10 uses multiple transducers 32each within a transducer housing 14, there are multiple chordal pathsdistributed in spacing according to numerical integration rules in orderto accurately sample the velocity profile in a pipe 12. A path consistsof one transducer 32 upstream from the gas flow and another isdownstream from the gas flow, both transducers 32 transmit and receivesignals. The difference in transit times between the upstream anddownstream signal is used to calculate a velocity per path. The pathvelocities are integrated by the controller 20 to calculate a flow rate.These equations can be modified for Mach number.

For C>>V:

L: path length

L_(chord): chord path length

v_(axial): axial gas velocity

Q: Volume flow

D: diameter of opening

φ: path angle

t₁: upstream transit time

t₂: downstream transit time

Δt: t₂−t₁

V_(path): gas velocity per path

w_(l); Weighting factor per path

Q = ∫∫_(cross  section)v_(axial)(x, y)d x  d y${L_{path}V_{path}} = {1\text{/}2\frac{L_{path}^{2}\Delta\; t}{t_{2}t_{1}}}$V_(path) = V_(axial)sin  φ $L_{path} = \frac{L_{chord}}{\cos\;\varphi}$${V_{axial}L_{chord}} = {1\text{/}2\frac{L_{path}^{2}\Delta\; t}{t_{2}t_{1}\tan\;\varphi}}$

In order directly to measure volumetric flow, one must integrate theaxial fluid velocity over a cross section normal to the pipe 12 axis. Inorder to solve for the speed of sound in gas and gas velocity, theupstream and downstream transit times need to be measured via acontroller 20. The controller 20 computes the transit time differencesbetween the upstream and downstream transit times per path length. TheV_(axial)L_(chord) product is exactly the line integral of V_(axial) dyat a chord location. The V_(axial)L_(chord) product is calculated foreach location x₁, x₂, x₃, x₄ in FIG. 3, effectively dividing the pipe 12cross-section into four segments per plane. The effective width of eachsegment is a fraction of the internal diameter, D, measured along the xaxis. Either Legendre or Jacobian/Chebychev spacing or weighting areused for chordal flowmeters, the path locations y, and weighting factorsw were not chosen arbitrarily but comply with numerical integrationrules. The spacing is measured from the center of the pipe 12, thespacing y₁, y₂, −y₁, −y₂ is shown in Table 1 along with the weightingfactors. The length of each chord is known either by calculation ormeasurement.

TABLE 1 Flowmeter Path Spacing and Weighting Factors Loca- Jacobian/Jacobian/ tion Legendre Legendre Chebychev Chebychev y-axis Spacingweighting Spacing weighting   y₁   .34 * Diameter/2 .77   .30*Diameter/2.72   y₂   .86 * Diameter/2 .22   .80*Diameter/2 .27 −y₁ −.34 *Diameter/2 .77 −.30*Diameter/2 .72 −y₂ −.86 * Diameter/2 .22−.80*Diameter/2 .27

The flow Q can now be calculated by the following equation:Q=D[w ₁ Lchord₁ vaxial₁ +w ₂ Lchord₂ vaxial₂ +w ₃ Lchord₃ vaxial₃ +w ₄Lchord₄ vaxial₄].

where w₁=w₄ and w₂=w₄; Lchord₁=Lchord₄, Lchord₂=Lchord₃.

A 24 inch diameter (21.56″ ID) ultrasonic gas flowmeter 10 with eightpaths was fabricated as described in this invention and calibrated to aknown standard at the CEESI gas calibration facility in Iowa. Resultsshow (FIG. 5) the percent error as function of a range of velocitiesfrom 2 ft/s to 100 ft/s. Native linearity (that is; the 24 inch meterlinearity without correction) was determined to be only +/−0.175%. Thepipe 12 is fabricated from carbon steel in accordance ASME B31.3 ProcessPiping Code. The controller 20 is designed in compliance to UL/cUL Class1, Division 1, Groups C & D.

The meter sizes, flowrates and velocities are described in Table 2.Flowrates shown are based on schedule 40 pipe ID. Over-range flowratesare at 120 fps.

TABLE 2 Meter Sizes, Flowrates, and Velocities Meter Size Flow Rate -ft³/hr Flow Rate - m³/hr Inches DN Min Max Over-range Min Max Over-range8 200 2,500 125,000 150,000 71 3,550 4,250 10 250 3,950 197,000 237,000110 5,580 6,700 12 300 5,600 280,000 336,000 160 7,900 9,500 16 4008,850 442,000 530,000 250 12,500 15,000 20 500 13,900 695,000 834,000400 19,700 23,600 24 600 20,100 1,010,000 1,210,000 570 28,500 34,200

FIG. 6 shows the transformer application. FIGS. 7a and 7b , whichtogether are one continuous drawing, show a demonstration of transittime flow meter performance. FIGS. 8a and 8b , which together are onecontinuous drawing, show error vs. velocity in regard to the claimedinvention.

Although the invention has been described in detail in the foregoingembodiments for the purpose of illustration, it is to be understood thatsuch detail is solely for that purpose and that variations can be madetherein by those skilled in the art without departing from the spiritand scope of the invention except as it may be described by thefollowing claims.

The invention claimed is:
 1. A flowmeter for detecting gas flow rates ina pipe comprising: a container configured to be attached to the pipehaving a channel through which the gas flows, and a plurality ofrecesses that extend through the container and a plurality of housings,each recess having a housing with a window that is virtuallyacoustically transparent; a plurality of transducers, with onetransducer of the plurality of transducers disposed in each housing ineach recess and coupled to the window, the transducers transmittingultrasonic signals into and receiving ultrasonic signals from thechannel; acoustic isolators made of plastic or foam discs whichacoustically isolate the housings from the container; and a controllerin electrical communication with the plurality of transducers whichdetermines the gas flow rate through the channel by measuring transittimes of signals transmitted by and received by the transducers.
 2. Theflowmeter as described in claim 1 wherein the ultrasonic signalstransmitted and received by the transducers define a first path in afirst plane and a second path in a second plane which paths cross in thechannel.
 3. A method for detecting gas flow rates in a pipe comprisingthe steps of: transmitting ultrasonic signals from a plurality oftransducers disposed in recesses in a container attached to the pipeinto a channel of the container in which the gas flows, each recesshaving a housing with a window that is virtually acousticallytransparent, with one transducer of the plurality of transducersdisposed in each recess and coupled to the window and with acousticisolators made of plastic or foam discs which acoustically isolate thehousings from the container; receiving ultrasonic signals from thechannel by the transducers in the recesses; and determining the gas flowrate through the channel by measuring transit times of the signalstransmitted by and received by the transducers with a controller inelectrical communication with the plurality of transducers.
 4. Themethod as described in claim 3 wherein the transmitting step includesthe step of transmitting ultrasonic signals by the transducers along afirst path in a first plane and a second path in a second plane whichcross in the channel and the receiving step includes the step ofreceiving ultrasonic signals by the transducers from the first path andfrom the second path.
 5. The method as described in claim 4 wherein thetransmitting step includes the steps of: generating with an upstreamultrasonic transducer of the first path plane waves that propagatethrough the channel and are received by a downstream ultrasonictransducer of the first path; producing a downstream transducer signalwith the downstream transducer from the plane waves the downstreamtransducer receives; generating with the downstream ultrasonictransducer of the first path plane waves that propagate through thechannel and are received by the upstream ultrasonic transducer of thefirst path; producing an upstream transducer signal with the upstreamtransducer from the plane waves the upstream transducer receives; anddetermining with the controller the gas flow rate from transit times ofthe signals generated and received by the upstream transducer anddownstream transducer.
 6. A flowmeter for detecting gas flow rates in apipe comprising: a container configured to be attached to the pipehaving a channel through which the gas flows, and a plurality ofrecesses that extend through the container and a plurality of housings,each recess having a housing; a plurality of transducers, with onetransducer of the plurality of transducers disposed in each housing ineach recess, the transducers transmitting ultrasonic signals into andreceiving ultrasonic signals from the channel; acoustic isolators madeof plastic or foam discs in direct contact with the pipe whichacoustically isolate the housings from the container; and a controllerin electrical communication with the plurality of transducers whichdetermines the gas flow rate through the channel by measuring transittimes of signals transmitted by and received by the transducers.